Download Introduction

BIM 310: Computer Networks
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•
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•
Time: Tuesdays 9-12am, Thursdays 11am-1pm
Location: BLab4
Instructor: Cuneyt Akinlar
Grading:
–
–
–
–
Midterm I – 20%
Midterm II – 30%
1 Final – 40%
Homeworks - 10%
1
High-level picture of the problem
Transmission System
Sender Node
Destination Node
Problem: Transmit a message M from a
source node to one or more destination
node(s) through a transmission
system=computer network
Nodes: Things that send/receive messages.
Examples are PCs, labtops, PDAs, Internet
telephones etc.
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Transmission System
Sender Node
Destination Node
• mesh of interconnected routers (switches) inter-connected in an
arbitrary topology
• the fundamental question: how is data transferred through net?
– circuit switching: dedicated circuit per call: telephone net
– packet-switching: data sent thru net in discrete “chunks”
3
Circuit-Switching: Idea
End-end resources reserved for “call”
• call setup required
• After the call, the resources (the circuit bandwidth) is dedicated
and is not shared with other calls
• circuit-like (guaranteed) performance
• This course is not about circuit-switching, but we will touch on it
so that you get an idea on how it works
4
Circuit-Switching: Data Flow
• After the call is setup, the data flows through the
circuit bit by bit
– No store or forward delay at the routers (switches)
– As soon as a bit from the connection arrives at a router, it is
immediately forwarded over the outgoing link without any delay
– So the transmission time is independent of the # of links from
the source to the destination
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CS: Sharing Link Capacity
• How do several calls using the same link share the link?
– In the example above, we have 2 calls sharing 2 links in the
middle of the network
• 2 Approaches
– Frequency Division Multiplexing (FDM)
– Time Division Multiplexing (TDM)
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Circuit-Switching: FDM
• FDM: Divide the link capacity into several
frequency bands (space-wise division) and
allocate each band to a different call
– Each circuit gets the fraction of the bandwidth
continuously (all the time)
– Also used by radio/TV transmission through the air
7
Circuit-Switching: TDM
• TDM: Divide the link capacity in time into
several slots and allocate each slot to a
different call
– Each circuit gets ALL of the link bandwidth
periodically
– Used by wireless telephones (GSM)
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Circuit Switching Example
• 1890-current: Phone network
–
–
–
–
–
Fixed bit rate
Mostly voice
Not fault-tolerant
Components extremely reliable
Global application-level knowledge throughout
network
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Circuit Switching: Summary
• Establish a dedicated circuit before sending data
–
–
–
–
Dedicated resources
Data flows through the circuit
No store and forward at the switches (routers)
Good for constant-bit-rate traffic such as voice
• BUT
– Dedicated resources means if no data is flowing the circuit,
the allocated resources are idle
• Leads to waste of network resources
• Might be OK for telephone calls where two parties are typically
talking all the time
• But is this good if two people are exchanging a data as in a instant
messaging session?
– How can be let other people use unused bandwidth?
• Packet Switching -- Next
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Packet-Switching: Idea
Message M
Message M
queue of packets
waiting for output
link
• Divide the message into smaller chunks, packets
• Send each packet through the network independently
• Each packet uses a link’s full bandwidth during
transmission
• Resources are used as needed
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Packet switching vs Circuit Switching
• 1 Mbit link
• each user:
– 100Kbps when “active”
– active 10% of time
• circuit-switching:
– can admit 10 users
N users
1 Mbps link
• packet switching:
– With 35 users,
probability > 10 active
less that .004
Conclusion: Packet switching allows more users to use network!
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Packet-Switching: Link Sharing
A
Message M
B
Message M
queue of packets
waiting for output
link
• resource contention:
– aggregate resource demand can exceed amount available
– congestion: packets queue, wait for link use
– store and forward: packets move one hop at a time
• A router must receive the whole packet before the packet can be forwarded
• After reception, queue the packet internally and have it wait its turn for the
output link.
• This is done by each router: Per hop forwarding
• Sequence of A & B packets does not have fixed pattern
– Called statistical multiplexing.
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Packet-Switching: Issues
• Two Fundamental Questions must be answered
in a packet-switched network:
1. What should the packet size be?
• Fixed-size or variable-sized packets?
• How big?
2. Should we establish an end-to-end path through the
network for the packets to flow?
• Yes: Virtual-Circuit Networks (X.25, Frame-Relay, ATM)
• No: Datagram Networks (the Internet)
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Packet-Switching: Packet Size
Source
R1
R2
Destination
0
5
10
15
• Consider a message M, that is 7.5*106 bits long, no message frag.
• Assume each link has 1.5Mbps bandwidth
• It takes (7.5*106 /1.5Mbps) = 5 seconds to move the message from
the source to the first switch (router) R1
• Another 5 secs to move M from R1 to R2
• Another 5 secs to move M from R2 to destination
• Total time: 5 + 5 + 5 = 15 seconds
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Packet-Switching: Packet Size
Source = S
0
5
5002
1
2
3
R1
1
2
3
R2
1
2
3
Destination = D
1
2
3
5000
• Assume now that we divide the message into 5000 packets, each
1500 bits long
• It takes 1 milisecs to move the 1st packet from S to R1
• But while the first packet is being moved from R1 to R2, we are also
moving 2nd packet from S to R1
• The first packet makes it to D in time 3ms, the 2nd packet is at R2
and 3rd packet is at R1 at this time
• Following this logic, the last packet makes it to D at time 5002 16
ms =
5.002 sec as opposed to 15 seconds
Packet-Switching: Packet Size
•
Small-sized packets has yet another advantage
–
Bit errors can be introduced as packet travels through the
network. In such cases the packet is simply discarded
•
•
The smaller the packet, the smaller the discarded info
If small packets are so good, why not make them 1 byte
–
–
–
Each packet carries some headers with it
Headers are used for packet forwarding, and other stuff
The smaller the packet, the bigger the header to payload ratio,
which translates to more waste of bandwidth
–
Consider 100 byte packets with 20 byte header
•
–
20% bandwidth waste
Consider 1000 byte packets with 20 byte header
•
2% bandwidth waste
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Virtual Circuits Networks: Signaling
A
R2
R3
R5
C
R10
R6
R1
B
R9
R4
R7
R8
• Virtual Circuit Networks (e.g., X.25, Frame Relay, ATM)
– Establish a path along which the packets will flow between the
source and the destination. How?
• Use a signaling (virtual circuit establishment) protocol
• Ex: B tells its router (R1) that it wants to talk to C
• The call establishment message is forwarded by the routers in the
network until it reaches C. Then a reply comes back from C to B.
– Path established at call setup time remains fixed during packet
exchange
– Routers maintain state information for ongoing connections 18
D
Virtual Circuits Networks: Forwarding
A
45
1
12
R5
R2
R3
3
2 53
2 22
1
R4
R1
43
2
9
69
66
R9
R7
B
D
77
R8
VC table at R1:
–
–
–
3
R10
R6
C
VC table at R2:
Incoming
Interface
Incoming
VC #
Outgoing
interface
Outgoing
VC #
Incoming
Interface
Incoming
VC #
Outgoing
interface
Outgoing
VC #
1
12
2
22
1
45
3
53
2
38
1
19
3
8
1
15
each packet carries tag (virtual circuit ID), which determines next hop
Path established at call setup time remains fixed during packet exchange
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Routers maintain state information for ongoing connections
Datagram Networks: Idea
A
D
C
B
R2
R3
D
R1
C
D
C
D
R5
R4
C
C
D
R10
C
R6
D
R7
D
C
C
D
C
C
R9
D
D
R8
• Datagram networks (e.g. the Internet):
• No call establishment before data exchange
• Simply put the destination address on top of the
packet and submit it to the network for delivery
• Similar to postal service
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Datagram Networks: Forwarding
A
R2
1
D
3
D
2
1
C
R3
R1
B
C
D
2 C
R4
R5
C
R6
D
C
R10
C
C
C
C
R9
R7
D
D
R8
Forwarding table at R1:
Forwarding table at R2:
Destination
Address
Outgoing
interface
B
A
1
A
2
R3
C
3
R3
2
R3
D
3
R3
Destination
Address
Outgoing
interface
B
1
C
D
Next
Hop
Next
Hop
– Destination address is written on top of a packet and it is simply
submitted to the network for delivery (like postal service)
– Routers look at destination address in packet to determine the next hop
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– No connection-state information needed in the routers
– Routes may change during session
Packet switching versus circuit switching
Is packet switching a “slam dunk winner?”
• Great for bursty data
– resource sharing
• But, excessive congestion: packet delay and loss
– protocols needed for reliable data transfer,
congestion control
• Q: How to provide circuit-like behavior?
– Bandwidth guarantees needed for audio/video apps
– Active research area: IP QoS
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Network Taxonomy
Telecommunication
networks
Circuit-switched
networks
FDM
TDM
Packet-switched
networks
Networks
with VCs
Datagram
Networks
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Packet Switched Networks
• This course is about packet-switched networks
• We will not cover circuit-switched networks
• In looking at packet-switched networks, our
approach will be from the view of network
designer, a system engineer, who wants to build
a packet switched network from the ground up
–
–
–
–
–
How do you build a packet switched network?
What are the issues?
How do you solve them?
What are the specific solutions in existence today?
We will mostly look at Internet Protocols
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Point-to-Point Links
A
B
Simple
point-to-point link
Message M
• The simplest packet switched network is a network
consisting of 2 hosts, A and B, and a link connecting
them
– Link can be guided media, i.e., a copper, coax, fiber wire
– Link can be unguided media, i.e., the air – wireless
– Link can be half-duplex (only one node can send data over the
link at any time) or full-duplex (A can send a message to B,
while B is sending a message to A)
• Problem: Given a message M at A, divide the message
into several packets, and send them over the link to B
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Point-to-Point Links
A
B
Simple
point-to-point link
Message M
• What are the issues in a point-to-point link?
– How does B know the beginning and end of a packet?
• Called the framing problem
– How does B know whether the packet is corrupted, i.e., if any
bits of the message has changed, during transmission or not?
If any bits changed, can B correct them?
• Called the error detection & correction problem
– How do you encode a digital data on the link?
• Called the data encoding problem
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Broadcast (Multi-Access) Links
B
A
Message M
C
D
• The next-simple packet switched network you can
imagine is a network consisting of several hosts, A, B, C
and D above, sharing a common link
– Again, the link can be wired or wireless
• In such a network, when one node sends a packet over
the link, the packet reaches ALL nodes attached to the
link
– Such a link is called a broadcast link, e.g., Ethernet, FDDI
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Broadcast (Multi-Access) Links
B
A
Message M
C
D
• What are the issues in a broadcast link?
– All the issues of a point-to-point exists: framing, error
detection & correction and encoding.
– What else? First issue is, how do the stations agree on who gets
to use the link?
• Called the media access control problem
– Second, how does A tell that the packet is destined to B not to
C or D?
• Addressing problem: Each station must have a UNIQUE address,
called the Media Access Control (MAC) address
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Limit on Broadcast Links
B
A
Message M
C
D
• What’s the limit of a broadcast link?
– How many hosts (stations) can we connect to a broadcast link?
– Can we build a global network such as the Internet with a
broadcast link?
• Can you imagine connecting millions of hosts to a broadcast link?
• If we do, does it make sense that when a host in Germany wants to
send a packet to another host next door, that my host in here
receive that packet, examine it, realize that the packet is destined
to someone else and discard it?
– Broadcast does not scale. So there is a limit on the size of a
broadcast link.
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A General Packet-Switched Network
A
R2
B
R3
R5
R10
F
R6
R1
C
R4
R7
D
A Broadcast
Link
E
Network Core
A Point-to-Point
Link
G
R8
H
I
• To build a global packet-switched network such as the Internet, we
must have a “network core” consisting of lots of packet switches,
called routers
• The end systems (hosts, stations) are at the edge of the network
• End system hosts can be attached to the network core with a pointto-point link or they can be attached together with a broadcast link
and then attached to the network core
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A General Packet-Switched Network
A
R2
B
R3
R5
E
R10
F
R6
R1
C
R4
R7
D
A Broadcast
Link
Network Core
A Point-to-Point
Link
G
R8
H
I
• What are the issues in such a packet-switched network?
– Addressing: Each host and router interface must have
GLOBALLY UNIQUE addresses – IP address
– When host A wants to send a packet to host F on the other side
of the network, how does A and routers know how to reach F?
• Routing and forwarding problem: Establishing reach-ability
information (forwarding table) and using it to forward a packet from
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the source to the destination host
A General View of the Internet
local ISP
Connection to
national ISP
router
server
workstation
mobile
regional ISP
a company
network
a university
network
Mesh of interconnected autonomous systems
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What about applications?
FTP
Client
Web
Browser
R2
R3
R5
Web
Server
C
R10
R6
A
R1
B
D
R9
R4
R7
FTP
Server
R8
• It is the applications that communicate!
–
–
–
–
Host A runs a Web Browser and an FTP Client
Web Browser is talking to the Web Server running C
FTP Client is talking to the FTP Server running in D
Packets from both C and D arrive at A
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What about applications?
Web
Browser
FTP
Client
R2
R3
R5
Web
Server
C
R10
R6
A
R1
R4
R7
B
D
R9
FTP
Server
R8
• What are the issues here?
– How does host A know that “green packets” need to delivered to
the Web Browser and Blue Packets need to be delivered to the
FTP client?
• Multiplexing/Demultiplexing problem
– What if some of the packets sent from the Web Server is lost
during transmission. How do we recover them?
• Reliable packet delivery problem
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How do two network entities talk to
each other: PROTOCOLS
• For two entities to communicate, they must
“speak the same language”
– What is communicated?
• Message format
– How is it communicated and what it means?
• Order of messages and their meaning
– When is it communicated?
• Timing of the messages
– The above must conform to mutually acceptable
conventions between the entities involved
– In networking, these conventions are referred to
as a “protocol”
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Protocols
• A protocol is a set of rules governing the
exchange of data between the two entities
• Key elements of a protocol are
– Syntax: Message format
– Semantics: The meaning of messages
• order of messages sent and received
• actions taken on message transmission, receipt
– Timing: Includes speed matching and sequencing
• When to send a message
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More on Protocols
a human protocol and a computer network protocol:
Hi
TCP connection
req
Hi
Got the
time?
2:00
TCP connection
response
time
Get http://www.awl.com/kurose-ross
<file>
• What are some other human protocols?
• Raise you hand before asking questions
• Take turns to speak, i.e., do not speak at the same time
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Designing Protocols
A
B
Message M
• Recall the issues in communication over a P-2-P link
–
–
–
–
Message fragmentation: Dividing a message into packets at A
Framing: Identifying the beginning and end of a packet at B
Error Detection & Correction: Identifying corrupt packets at B
Encoding: Encoding packet bits onto the link as a signal at A and
reconstructing the packet bits from the received signals at B
• What kind of protocols do we need to handle the above
issues?
– Next
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Physical and Link Layers
Message M
A
link
physical
link
physical
B
• Typically the listed issues are handled by 2 protocols
– A Physical Layer (PL), which deals with bit Encoding/Decoding
• Physical Layer at A deals with the following problem:
– Given a sequence of bits (bits making up a packet), how do you encode
the bits onto the link as signals (electromagnetic, light..)
• Physical Layer at B deals with the following problem:
– As you receive signals from the link, how do you decode these signals
into bits?
– A Link Layer (LL) that sits on top of the physical layer (PL) and
deals with the rest of the problems: Message Fragmentation,
Framing, Error Detection/Recovery
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Physical and Link Layers
Message M
A
link
physical
link
physical
C
link
physical
B
• What about a broadcast link?
– PL and LL will be there as before having the same
responsibilities as described before
– But now LL has the additional responsibility of Media Access
Control (MAC) to deal with
• Link Layer: data transfer between neighboring network elements
– PPP, Ethernet
• Physical Layer: bits “on the wire”
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Network Layer
Network
link
Link
physical
Physical
A
Network
Link
link
Physical
physical
R1
Network
Link
link
Physical
physical
R2
Network
link
Link
physical
Physical
B
• What about a general packet-switched network?
– Are PL and LL enough?
– Recall that LL is responsible for data transfer between
neighboring network elements, that is, if they are connected to
the same link
• Are hosts A and B neighbors above? No.
– Need a new layer, called the “Network Layer”
• Responsible for forwarding of datagrams from source HOST A to
destination HOST B
• Internet Protocol (IP, routing protocols)
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Web
Browser
FTP
Client
Transport Layer
Transport
Network
Network
Link
Link
Physical
Physical
A
Network
Link
Physical
R1
Web
Server
Transport
Network
Network
Link
Link
Physical
Physical
Network
Link
Physical
R2
C
FTP
Server
B
Transport
Network
Network
Link
Link
Physical
Physical
• What about applications running in hosts?
– Is NL enough?
• Recall that NL is responsible for forwarding a packet from one HOST
to another HOST
• How do you make applications on HOSTs to communicate?
– Need a new layer, called the “Transport Layer”
• Responsible for providing communication between applications running
in different hosts
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• A Web Browser talking to a Web Server
Web
Browser
FTP
Client
Application
Transport
Transport
Network
Network
Link
Link
Physical
Physical
A
Application Layer Application
Web
Server
Network
Link
Physical
R1
Network
Link
Physical
R2
C
Transport
Network
Link
Physical
FTP
Server
Application
B
Transport
Transport
Network
Network
Link
Link
Physical
Physical
• Lots of different applications in the Internet
– Web browsing, file download, e-mail, instant messages, presence…
– Each require different message types, formats, actions
• So need a new layer, called the “Application Layer”
– Responsible for defining application specific message types,
formats, actions taken on messages
– HTTP for Web, FTP for file download, SMTP for e-mail, SIP for
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instant messaging and presence… so many others!!
Internet protocol stack
• application: Define application specific
message types, formats
– FTP, SMTP, STTP
• transport: Provide application-to-application
communication
– TCP, UDP
• network: Provide host-to-host
communication. That is, forwarding of
packets from source to destination
– IP, routing protocols
• link: Provide data transfer between
neighboring network elements (host-tohost, host-to-router, router-to-router)
application
transport
network
link
physical
– PPP, Ethernet
• physical: transmit bits “on the link”
44
Protocol layering and data
• At the source, each layer takes data from above
– adds header information to create new data unit
• Called “encapsulation”
– passes new data unit to layer below
• At the destination, each layer takes data from below
– strips off its own header
• Called “decapsulation”
– passes the remaining part of the packet to the upper layer
source
M
Ht M
Hn Ht M
Hl Hn Ht M
application
transport
network
link
physical
destination
application
Ht
transport
Hn Ht
network
Hl Hn Ht
link
physical
M
message
M
segment
M
M
datagram
frame
45
Multiplexing/Demultiplexing
• A way for multiple protocol
objects at one level to
identify themselves to the
protocol above or below
them.
MUX
1
1
data
– Multiplex
3
data
DEMUX
• Tag each message with a key
– Lower protocol knows where it
came from!
3
2
1
1
3
2
data
3
data
– Demux
• Use key on arriving packet to
know where to send it above
1
data
3
data
46
Protocol Interfaces
• Each protocol defines 2 interfaces
– Service Interface: The kind of services it provides to
protocols that sit on top of it on the same machine
– Peer Interface: Communication interface with its
counterpart (peer) on another machine
• This interface defines the form and meaning of messages
exchanged between protocol peers to implement the service
interface
Host 1
Upper-layer
protocol
Protocol
Host 2
Service
interface
Peer-to-peer
interface
Upper-layer
protocol
Protocol
• Example: IP
exports a
connectionless,
unreliable, besteffort datagram
service to
transport layer
protocols
47
Protocol Communication
Link Layer
Link Layer
A protocol always communicates with same
protocol at peer machine. Never do we have a
protocol at one layer talk to another protocol at
a different layer at the peer
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Internet Protocols
•
•
•
•
•
•
Defined by Internet Engineering Task Force (IETF)
Hourglass Design: Everything goes over IP
Lots of application layer protocols
Mainly 2 transport layer protocols: TCP, UDP
Network Protocol is Internet Protocol (IP)
Any Link Layer Protocol
FTP
HTTP
NV
80
20,21
RTP
4444
UDP
TCP
6
17
IP
NET1
NET2
…
NETn
49
OSI v TCP/IP
• Open Systems
Interconnection
– Developed by the International
Organization for
Standardization (ISO)
– Seven layers
– A theoretical system delivered
too late!
– TCP/IP is the de facto standard
50
Network Performance Metrics
• Bandwidth
–
–
–
–
data transmitted per time unit
link versus end-to-end
Notation: Mbps = 106 bits per second
Bits transmitted at a particular bandwidth can be
regarded as having some width
(a) Bits transmitted at 1Mbps, each bit is 1us wide
(b) Bits transmitted at 2Mbps, each bit is 0.5us wide
1 second
(a)
(b)
51
Network Performance Metrics
• Latency (delay)
– time for the first byte of the message to reach the destination
– one-way versus round-trip time (RTT)
– components
Latency = Transmission + Propagation + Nodal Pros. + Queuing Delay
Transmission Time = Message Size / Bandwidth
Propagation = Distance / Speed of Light
Nodal Processing = F(Amount of Processing, Processor Speed)
Queuing Delay = F(Amount of total traffic)
transmission
A
transmission
B
nodal
processing
queueing
C
52
Latency or Delay
d delay  d trans  d prop  d processing  d queuing
• dtrans = transmission delay
– = L/R, L: Message size, R: Link Bandwidth
– significant for low-speed links
• dprop = propagation delay
– a few microsecs to hundreds of msecs
• dproc = processing delay
– typically a few microsecs or less
• dqueue = queuing delay
– depends on congestion (the amount of total traffic)
53
Queueing delay
• R=link bandwidth (bps)
• L=packet length (bits)
• a=average packet
arrival rate
traffic intensity = La/R
• La/R ~ 0: average queueing delay small
• La/R -> 1: delays become large
• La/R > 1: more “work” arriving than can be
serviced, average delay infinite!
54
How and why do packet loss occur?
• Packets get queued in router buffers
• If packet arrival rate exceeds output capacity, packets
get buffered and wait for their turn to be transmitted
• Buffer is of finite size
– If more packets than what buffer can store, new packets will
be dropped
packet being transmitted
A
B
packets queueing
free (available) buffers: arriving packets
dropped (loss) if no free buffers
55
Introduction: Summary
Covered a “ton” of material!
• Internet overview
• what’s a protocol?
• network edge, core
– packet-switching versus
circuit-switching
• performance: loss, delay
• layering and service models
You now have:
• context, overview,
“feel” of networking
• The rest of the
course will be to learn
the details of these
protocols in the
context of IP protocol
stack
56